Wednesday Colloquium


"Stability-Causality Theorems"

Lorenzo Gavassino (CAMK, Warsaw)

Relativistic hydrodynamics is tricky. There are pitfalls at every corner. Many authoritative textbooks, which are currently used as standard references both in physics and astronomy, present several hydrodynamic equations that lead to completely unphysical predictions (e.g., a glass of water should spontaneously detonate in 10^-34 seconds!). Among such books, we find Novikov-Thorne ("Astrophysics of black holes"), Misner-Thorne-Wheeler ("Gravitation"), Mihalas-Mihalas ("Foundations of radiation hydrodynamics"), and many others. Hence, it is not surprising that such incorrect equations have become part of the "common sense" of a considerable fraction of the astrophysical community, being used both in theoretical models and numerical simulations. How bad is the situation? For decades, some of the brightest minds (e.g. Carter, Israel, Lindblom, Geroch, Anile, Ruggeri) have been looking for more reliable equations, but nobody ever analyzed the problem in full generality. What makes a theory problematic? What makes a theory reliable? What happens if we simulate the wrong theory? Is there an intuitive explanation for all of this? I will provide the definitive answer to all these questions, and I will do it without writing a single equation.


"A multitude of Multiverses: what is "Multiverse"?"

Łukasz Lamża (Copernicus Center for Interdisciplinary Studies, Jagiellonian University)

The concept of "Multiverse" enjoys varying popularity in contemporary cosmology, with some physicists considering it a viable explanation of cosmological facts, and other denying it any claims for scientific nature. Here, a simple typology of Multiverses is presented to help in organizing and understanding the wealth of studies on the subject: 1) non-cosmological Multiverse (e.g. Everett's many-worlds interpretation of quantum mechanics, Susskind's string theory landscape); 2) cosmological, mathematical Multiverse (e.g. Carter's ensemble of Universes, Tegmark's level 4 Multiverse); 3) cosmological, physical Multiverse (e.g. Tolman's cyclic model, Linde's chaotic inflationary multiverse, Smolin's cosmological natural selection, Penrose's conformal cyclic cosmology). Their theoretical and observational status is shortly summarized.


"Magnetic field in neutron stars: from the interior to the exterior"

Ankan Sur (CAMK, Warsaw)

The origin of the magnetic field in neutron stars, which has strengths trillions of times stronger than terrestrial magnets, remains a mystery to date. To unravel this mystery, modeling the magnetic field and understanding its equilibrium are critical. A key step towards this understanding is magnetohydrodynamics (MHD) studies. In this talk, I will discuss the results of our MHD simulations from which we had obtained various magnetic field configurations. While these results come from time-evolving systems, I will also discuss equilibrium solutions from the Grad-Shafranov equation for a normal matter crust and superconducting core neutron star. These results are applicable for the standard pulsar population. Modeling the emission from neutron stars, such as gravitational waves or electromagnetic waves, is another important step towards understanding the magnetic field. As an example, I will show how a newly millisecond magnetar may form accretion "mountains" and emit gravitational waves. And lastly, I will also discuss how the geometry of the magnetic field in pulsars can be constrained based on radio observations.